Reverse border radius with css3 [duplicate]

Answer 1

If your tab has a solid background color you can use pseudo elements to archive this.

---

First we create a tab with border radius in the top left and top right corners.

.tab {
    -webkit-border-top-left-radius: 10px;
    -webkit-border-top-right-radius: 10px;
    -moz-border-radius-topleft: 10px;
    -moz-border-radius-topright: 10px;
    border-top-left-radius: 10px;
    border-top-right-radius: 10px;
}

Then we add position: relative, because we need to use position: absolute in the pseudo elements.

.tab {
    position: relative;
}

Now, we are going to create and position our pseudo elements as a quarter of a circle, transparent, with box-shadow to simulate borders.

.tab:before,
.tab:after {
    content: "";
    position: absolute;

    height: 10px;
    width: 20px;

    bottom: 0;
}

.tab:after {
    right: -20px;

    border-radius: 0 0 0 10px;
    -moz-border-radius: 0 0 0 10px;
    -webkit-border-radius: 0 0 0 10px;

    -webkit-box-shadow: -10px 0 0 0 #fff;
    box-shadow: -10px 0 0 0 #fff;
}

.tab:before {
    left: -20px;

    border-radius: 0 0 10px 0;
    -moz-border-radius: 0 0 10px 0;
    -webkit-border-radius: 0 0 10px 0;

    -webkit-box-shadow: 10px 0 0 0 #fff;
    box-shadow: 10px 0 0 0 #fff;
}

And this is the result:

DEMO: jsFiddle

body {
	background: #1c1c1c;
	padding: 50px;
}

.tab {
	float: left;

	width: 90px;
	padding: 10px 15px;
	height: 30px;

	position: relative;

	background: #fff;

	-webkit-border-top-left-radius: 10px;
	-webkit-border-top-right-radius: 10px;
	-moz-border-radius-topleft: 10px;
	-moz-border-radius-topright: 10px;
	border-top-left-radius: 10px;
	border-top-right-radius: 10px;
}

.tab:before,
.tab:after {
	content: "";
	position: absolute;

	height: 10px;
	width: 20px;

	bottom: 0;
}

.tab:after {
	right: -20px;

	border-radius: 0 0 0 10px;
	-moz-border-radius: 0 0 0 10px;
	-webkit-border-radius: 0 0 0 10px;

	-webkit-box-shadow: -10px 0 0 0 #fff;
	box-shadow: -10px 0 0 0 #fff;
}

.tab:before {
	left: -20px;

	border-radius: 0 0 10px 0;
	-moz-border-radius: 0 0 10px 0;
	-webkit-border-radius: 0 0 10px 0;

	-webkit-box-shadow: 10px 0 0 0 #fff;
	box-shadow: 10px 0 0 0 #fff;
}

<div class="tab"></div>